1. Field of the Invention
The present invention relates to a data transmission method and a data transmission device. Specifically, the present invention relates to a data transmission method and a data transmission device that can be applied to a power line carrier communication modem and the like for high speed data communication using a power line, wherein a leak electromagnetic field (electromagnetic wave) that is radiated from the power line at the time of signal transmission can be suppressed. That is, the present invention relates to a data transmission method and a data transmission device that have a function of suppressing a leak electromagnetic field to reduce a noise applied to other receivers, while signal transmission is performed therethrough.
2. Description of the Related Art
FIG. 1 shows an example of a configuration of a power line carrier communication system. In this example, a power line includes a high-voltage distribution power line 9-2 of 6.6 kV provided between a power distribution transformer substation 9-1 and a pole transformer 9-3, and a 100V/200V low-voltage power distribution line 9-4 and a service line 9-5 provided between the pole transformer 9-3 and a house 9-6.
In this power line carrier communication system, an optical fiber is provided between an access node 9-11 of the power distribution transformer substation 9-1 and a modem inside the pole transformer 9-3 so as to enable data transmission with an optical signal. Further, data transmission can be performed between the pole transformer 9-3 and a modem of the house 9-6 whose plug is inserted into an outlet of the house 9-6, via the low-voltage power distribution line 9-4, the service line 9-5, and wiring 9-7 in the house 9-6.
Plural home electric appliances are connected to the low-voltage power distribution line 9-4, the service line 9-5, and the wiring 9-7, so that switching power sources and inverter circuits of these home electric appliances randomly generate noises to deteriorate communication quality in the above data transmission. For this reason, the following techniques are applied. That is, an FM modulation method, an FSK modulation method, a PSK modulation method, a spectrum diffusion method, and the like that are robust against noises are used. Alternatively, a multi-carrier modulation method, OFDM (Orthogonal Frequency Division Multiplexing), and the like are introduced to perform communication while avoiding use of carrier bands having many noises.
Meanwhile, in such a power line carrier communication system, a radiated leak electromagnetic field generated from the power line at the time of the signal transmission can adversely affect broadcasting media or other communications. Particularly, the noises can adversely affect a receiver for short-wave broadcasting to deteriorate quality in a voice of the broadcasting.
In order to reduce deterioration of the quality of the other communications caused by the radiated leak electromagnetic field, the transmission level in the power line carrier communication may be lowered. However, when the transmission level in the power line carrier communication is lowered, communication quality in the power line carrier communication is substantially lowered by noises generated from the switching sources and the inverters of the various home electric appliances.
Furthermore, there are many branching points on the power line. Accordingly, multi-paths (that are described hereinafter) are formed, so that a transmission signal of a particular frequency is reflected at the branching point, and the reflected signal is returned to the transmitter with the reflected signal having the opposite phase. FIG. 2 shows an equivalent circuit of the power line in which the reflected waves are generated by the multi-paths. In FIG. 2, the reference number 10-1 designates a transmission unit of the modem of the pole transformer, and the reference number 10-2 designates a reception unit of the modem of the house. The pole transformer modem transmission unit 10-1 is connected to the house modem reception unit 10-2 by the power line 10-3 having many branching points.
The “n” number of reflected waves that are generated by the reflection at many branching points spend respective delay times Δt1 through Δtn, and then are returned to the power line at the side of the pole transformer modem transmission unit 10-1 to be combined. With respect to a transmission signal of a certain frequency, the reflected wave and the carrier wave of the transmission signal overlap to generate a point where a voltage value becomes zero, that is, a point where the impedance becomes zero. Accordingly, at this point, a large current signal flows to generate a large leak electromagnetic field.
Furthermore, in the power line carrier communication system, a low-voltage power distribution line can function as an inductor for the pole transformer modem, and a service line and wiring in a house can function as a condenser for the pole transformer modem. In addition, a noise preventing condenser of each home electronic appliance connected to the wiring in the house is provided between AC 100V lines, so that each home electric appliance generates a large capacitance load.
Accordingly, for the pole transformer modem, the power line can function as a series resonance circuit including an output impedance, i.e., including R, L, and C. FIG. 3A shows this equivalent circuit of the power line from the viewpoint of the pole transformer modem. FIG. 3B shows frequency characteristics of a signal current flowing in the power line.
In FIG. 3A, R designates the output impedance of the pole transformer modem, L designates an inductor of the low-voltage power distribution line, and C designates a condenser produced by the service line and the wiring in the house. R, C, and L constitute the series resonance circuit. The series resonance circuit takes the lowest impedance when a frequency is the resonance frequency f0=1/{2π√{square root over ( )}(L−C)}. As shown in FIG. 3B, the signal having this resonance frequency causes a large current to flow, so that a large leak electromagnetic field wave is generated.
Furthermore, the power line becomes a distributed constant circuit, and branching circuits form a plurality of series resonance circuits that have different resonance frequencies, respectively. FIGS. 4A and 4B show a plurality of resonance points in the range of the frequency band of the transmission signal. In other words, FIGS. 4A and 4B show the change in the impedance and the flowing current that depend on a frequency value. As shown in FIGS. 4A and 4B, the impedance takes minimum values at several points indicated by the arrows in FIGS. 4A and 4B. Accordingly, at the minimum values of the impedance, the current takes maximum values to generate a large leak electromagnetic wave.
Furthermore, each branching circuit functions as an antenna. For example, when the resonance frequency is 30 MHz, the transmission speed of the radio wave becomes 3×108 [m/s], and the wave length of the radio wave becomes 10 m. As shown in FIG. 5, a node is generated at intervals of the half wave length that is 5 m. At each node, the maximum current or the maximum voltage is generated.
In addition, frequencies having values that are integral multiples of the resonance frequency also cause the resonance. Accordingly, large leak electromagnetic fields are generated at intervals of half wave lengths of these resonance frequencies. Since the electric home appliance has capacity load, a large electromagnetic field is generated at intervals of the half wave length of the resonance frequency larger than about 100 kHz.